\begin{tikzpicture}[scale=1]
\coordinate (O) at (0,0);
\coordinate (q) at (0.5,0);
\coordinate (-q) at (-0.5,0);
\coordinate (p) at (xyz polar cs:angle=25, radius=3.5);
\draw[->] (-6,0) -- (6,0) node[above] {$z$};
\draw[->] (0,-2) -- (0,2);
\draw (O) circle (1.5cm);
\draw[<->] (O) -- (xyz polar cs:angle=250, radius=1.5) node[near end,sloped,above]{$a$};
\filldraw[black] (4.5,0) circle(2pt) node[below right]{$-Q$};
\draw[->,shorten >=2pt] (O) -- (4.5,0) node[near end,above]{$\ve R_-$};
\filldraw[black] (-4.5,0) circle(2pt) node[below left]{$+Q$};
\draw[->,shorten >=2pt] (O) -- (-4.5,0) node[near end,above]{$\ve R_+$};
\filldraw[black] (-q) circle(2pt) node[below left]{$-q$};
\filldraw[black] (q) circle(2pt) node[below right]{$+q$};
\draw (2,0) arc (0:25:2.0cm) node[below right]{$\theta$};
\draw[->,shorten >=2pt] (O) -- (p) node[midway, above]{$\ve r$};
\filldraw[black] (p) circle(2pt) node[above
  right]{$P$};
\draw[dotted] (-q) -- (-0.5,0.8);
\draw[<->] (-0.5,0.8) -- (0,0.8) node[midway,above]{$d'$};
\end{tikzpicture}%
